Smuggling of various kinds of contraband has become a widespread problem in the context of global trade and international travel. This issue is particularly difficult to manage in case of import/export of large maritime cargo containers, where it is feasible to physically inspect only a small fraction of the bulk quantity of targets.
In pursuit of a solution to this problem, various passive and active detection techniques have been developed. The passive detection techniques involve identifying natural radioactive emissions emanating from objects of interest. Such passive techniques can be used to detect various types of nuclear materials, but may not be useful in detecting other types of contraband (e.g., illicit materials like drugs, etc.) In contrast to passive detection, the active detection techniques involve interrogating the objects with high energy neutrons and/or photons to ascertain the material distribution and composition of the objects.